Homebuilt CNC Router #2: Simpler, Stronger, Prettier

I needed to address some of the problems that I noticed from before. Complexity needed to be reduced. Some of the racking and shifting needed to be addressed. I wanted to remove the concept of keeping all the skate bearings so tightly pressed against the rails. And it needed a face lift. So I came up with a new (is anything new?) design.

I kept the leadscrew and motor combination along with the torsion boxes. I got rid of the box below the gantry to let the bed lay flat on the table top. The sides were to lose the stiffeners by using a different strengthening technique. The wrap-around Y carriage was to turn into a flat plate. And I went with V-bearings for the linear rails. These bearings are actually fancier than they look. They have two rows of ball-bearings to be able to take both lateral and radial pressure.

They were to ride on angle iron as the official V-rails were beyond my pocket book.

I spent a fair amount of time trying to find a way to make the gantry wall stronger. I actually tested various plys and MDF combinations. I found out that 3/4 inch MDF skinned with laminate on both sides yielded a very strong panel. I also found that yellow glue was better than contact cement at holding it all tight. This combination actually makes a small torsion box, which is a substrate tightly coupled to skins on both sides. I edge banded them in maple to keep out moisture.

My gantry now looked like this:

I went with dual motors on the long axis to virtually eliminate any racking and more tightly couple the drive mechanism to the bearings. I used wooden standoffs to attach the motors.

The leadscrews where coupled to the anti-backlash nuts with wooden holders on each side of the gantry.

I attached the Y leadscrew to the carriage underneath the Y torsion box to get it out of the way.

I used the removable Z from the last machine and mounted it on this one.

And then added dust collection. A must needed addition.

It was all working pretty darn alright. But there were a few things that I decided to upgrade. The V-bearings for the Y carriage were digging into the edge of the angle iron. I solved this by adding a few wooden strips and mounted the angle iron with the angle facing up. This vastly improved its ride. I had also noticed that sometimes the steel rods and brass bushings for the Z were sticking, so I switched them over to V-bearings as well.

And finally I cut the plate across the front of the machine and added an end vise for vertical board routing.

To those who are still hanging in there: On a different note, before I had the money to buy a CAM program, I needed something to use for test cutting. So I wrote SpiroCNC. I think alcohol was also involved. It would generate G-code for spirographs and other geometric designs. It was kind of stupid fun and is still used a bit by people in that same situation. I did also use it to generate some rosettes.

Gary, they are stepper motors so there is no feedback or encoders. Servos are a bit too much for me to handle. The machine is not allowed to miss a step, and it doesn’t. Those motors are really strong. I can barely hold back the gantry when it is moving. They have about 300 oz/inch of torque and 200 steps per revolution. I am micro-stepping them with 16 pulses per step. Each revolution is 1/4 inch (4 tpi), so that is 12,800 pulses per inch. I am still amazed that it all works. The controller is a kit from HobbyCNC.com.

My rapids are set at 150 ipm, or 2.5 inches per second. This is the rate that I can move the router around when I’m not cutting anything. The cutting rate is set by the CAM software depending on what conditions are; depth of cut, type of wood, bit RPM, diameter of the bit, etc. Just like hand routing. I am pretty chicken about pushing it, but I could take it up to the rapid rate if I wanted. Doing something like a mortise at 60 ipm or 1 inch/sec is still impressive to watch, for me. I cut something like that with successive cuts, dropping the Z about 3/8 inch per pass. So the simple answer is about as fast as you would cut it by hand.

Great website. Thanks for the link. Seriously thinking about this. A few questions.

1. Are you plugging this into your parallel port on your PC, or a USB port with a parallel adapter?2. Does the CAM software just output g-code and then you use a different program to send the code to the controller?3. Do you know of any software that can take a dwg or dxf file and use that to create g-code?

Too bad about no encoders. If they were being used by the controller it would allow the steppers to skip and not lose their position. I worked with servos in the automation industry and I guess I an kind of spoiled.

Gary

-- Gary - Never pass up the opportunity to make a mistake look like you planned it that way - Tyler, TX

You really have to ask yourself why. What do you want to do with it and how much do you want to spend in dollars and time. For me, I just wanted to do it, and fell into it. I had no expectations or real cost targets. That is probably not a good way to go for most people. I saw it more like the guys who restore an old car. You can’t ask them why or how much. They just want to do it. They spend a bundle and only drive it on Sundays, but I admire them for it. Building it drove my uber practical Dad and my artsy Sister crazy. The first argued that I didn’t need it, and the second argued that anything that it produced would be just look machine made anyway. I am tired of those arguments.

What is the cutting envelope? I haven’t really measured it. The basic size is about 2’ x 4’ because that is what fit in my shop. It can get kind of expensive to keep increasing the size, just like a boat. A lot of newbies want to cut 4×8 sheets, and they soon find out that will cost a lot of money. Remember flex and whip?

My stepper driver card and controller (Mach3) combination uses Windows XP and a parallel port on a PC of at least 1Ghz. I guess that rules out a lot of modern PCs, or you just buy a cheap PCI to parallel port adapter. USB is talked about a lot, but for the DIY it is not there yet. I am pretty sure that a USB to parallel dongle won’t cut it.

The main purpose of a CAM program is to convert DXF, EPS, and other line art type files (for 2D) to a G-code file. The fancier ones also help you pick feeds and speeds along with a simulation. I do this upstairs, copy the G-code file to a flash stick, carry it downstairs and feed it into the machine. A .dwg file is more like a .doc file. It is a design file that contains much more info than is needed. So you export it to DXF from your CAD program. For 3D (think carving) the CAM input file is more like a mesh file. The output is still G-code. Go to Vectric.com and download their demo. It was originally a sign making CAM, but it has a user friendly look and feel and you can get a better idea how this all works.